Conductive adhesives for high-frequency applications

In this paper, we present results from measurements and simulations on epoxy-based anisotropic electrically conductive adhesive joints (ACA). We have studied two different types of connections, flip-chip bonded Si test chip and a simple transmission line gap bridged by a copper foil. The test chips were mounted on three different substrates, a rigid FR-4 board, a flexible board and a high-frequency teflon-based duroid substrate. Equivalent electrical models are discussed based on physical considerations and the parameters were then fitted to measurement data in the high-frequency CAD tool HP MDS, A HP8510 network analyzer was used to measure S-parameters on rigid FR-4 and flexible boards in the frequency range 500 MHz-8 GHz for both flip-chips and bridges. For the duroid mounted flip-chips and bridges, the frequency range was 1-30 GHz. The microstructures of the ACAs were studied by cross-sectioning and then using scanning electron microscopy (SEM). For low frequency applications (500 MHz-8 GHz) the results indicated that the ACA flip-chip joint and the bridge joint mounted on the FR-4 board or flexible board can be used. For high-frequency applications (1-30 GHz) the results showed that the ACA flip-chip joint and the bridge joint mounted on the duroid substrate can be used. For duroid and FR-4 mounted flipchip joints and bridge joints ACA has proved to be as good as or even better than the solder joint, for FR-4 in the frequency range 45 MHz-2 GHz and for the duroid substrate 1-30 GHz.